Block ciphers operate on large blocks of digits with a fixed, unvarying transformation.
Stream ciphers represent a different approach to symmetric encryption from block ciphers. The seed value serves as the cryptographic key for decrypting the ciphertext stream. The pseudorandom keystream is typically generated serially from a random seed value using digital shift registers. In practice, a digit is typically a bit and the combining operation is an exclusive-or (XOR). Since encryption of each digit is dependent on the current state of the cipher, it is also known as state cipher. In a stream cipher, each plaintext digit is encrypted one at a time with the corresponding digit of the keystream, to give a digit of the ciphertext stream. The operation of the keystream generator in A5/1, an LFSR-based stream cipher used to encrypt mobile phone conversations.Ī stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream ( keystream).
